Electrochemical cell stacks such as fuel cell stacks are formed with a repeating series of thin flat plates. Each cell in the stack has plate components, and the individual cells must be separated from each other by impervious separator plates which prevent reactant leakage from one cell to the next. In a fuel cell, the plates must be formed from an electrically conductive material, and the preferred material is a carbon-filled resin which is molded in a press into the plate form. The plates are relatively thin, for example, from about 0.030 inch to about 0.100 inch in thickness, and are fragile. From an economy standpoint, the greater the area of the plates, the less expensive the power plant would be because fewer plates would be needed to produce a stack with any given square footage of electrode surface. At present however, size limitations on the plates are imposed by press capabilities, material shrinkage during forming and the difficulty in handling larger size thin fragile plate components. Another problem which arises when producing larger unitary plates relates to scrap. When a larger plate is found to be flawed, the entire plate must be scrapped. This problem thus tends to increase costs incurred in using the larger plates when the objective of the larger plates is to reduce costs. At the present, the largest plates which can be feasibly produced are square in shape and about four feet on a side.
Graphite separator plates are formed for use in electrochemical cells in accordance with this invention by forming smaller component plates and then edge bonding them together to form larger plates for use in the cell stack. These plates must maintain their integrity in a phosphoric acid environment of the acid fuel cell, and must also be uneffected by the high operating temperatures of electrochemical cells. In view of the fact that the plates are quite thin, the edge joint between adjacent component plates is a scarfed cut, tapered at an angle in the range of about 15.degree. to about 45.degree.. The scarf angle is preferably kept as small as possible, particularly when the plates are in the lower thickness range. The surface on the scarf cuts should display a controlled roughness which allows the adhesive to form a sturdy mechanical bond between adjacent plates without requiring an overly thick layer of adhesive. A 32 microinch finish on the scarf cuts has been found to provide a suitably rough surface to allow the formation of a strong bond between adjacent plates. Care must be taken not to make the scarf cuts too rough lest leakage paths be formed across the joint. The surface finish on the scarf cuts should be maintained in the range of about 16 microinch to about 63 microinch. The adhesive used to bond the plates together may be substantially any fluoropolymer resin which is activated by heat. The resin is coated onto the scarf surfaces and the latter are pressed together under heat and pressure to bond the joint. The resin actually melts and fills all of the interstices on the scarfs during the bonding procedure whereby a mechanical bond is formed. One particular resin which has been found to be satisfactory is a fluoropolymer sold by Dupont under the Trademark FEP 120.
It is therefore an object of this invention to provide an improved plate component for an electrochemical cell stack.
It is a further object of this invention to provide a plate component of the character described which has a larger surface area so as to lower the cost of manufacturing the stack.
It is an additional object of this invention to provide a plate component of the character described which is formed from a plurality of smaller plate components joined together edgewise.
It is another object of this invention to provide a plate component of the character described wherein the edge joint between the plate components are scarfed cuts having suitably rough surfaces to promote secure bonding with a heat activated resinous adhesive.
It is yet another object of this invention to provide an improved plate of the character described which the adhesive is a fluoropolymer resin.
These and other objects and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of a composite plate formed in accordance with this invention when taken in conjunction with the accompanying drawing.